An electrical communications circuit comprising a power supply, a trunk section, a spur section, isolation means disposed between the trunk section and the spur section, diagnostics apparatus connected to the trunk section and a field device connected to the spur section, in which the field device is adapted to send data signals over the communications circuit, which data signals comprise one or more core functions, in which the communications circuit and/or the data signals function under one or more pre-determined normal operating parameters, in which the diagnostics apparatus is adapted to detect one or more pre-determined breaches of said normal operating parameters, in which a spur diagnostics module is connected to the spur section and is adapted to monitor electrical characteristics of the spur section, in which the spur diagnostics module comprises interference means adapted to interfere with the communications circuit and/or the data signals sent thereon such that said one or more pre-determined breaches of the normal operating parameters are detectable by the diagnostics apparatus, but such that said one or more core functions of the data signals are not effected, and in which the spur diagnostics module is adapted to activate said interference means when it detects one or more pre-determined electrical events in the spur section.